1. Field of the Invention
The present invention relates in general to an automatic transmission of a motor vehicle, and more particularly to an improvement in a planetary gear train of the transmission.
2. Description of the Prior Art
Hitherto, there has been proposed a four-speed (viz., three forward speeds and one overdrive) automatic transmission the planetary gear train of which comprises two planetary gear units, a plurality of clutch units and a plurality of brake units. However, in the conventional planetary gear trains hitherto proposed, some part is forced to rotate considerably faster than the input shaft of the transmission under a certain speed condition.
As is known, the presence of such high speed part in the transmission is undesirable because of necessity of using a highly durable and thus expensive materials for the part and its surrounding parts. In addition to this, the presence of the high speed part tends to induce the following problem in the transmissions of a type wherein the high speed part is incorporated with a clutch unit to rotate with the clutch drum of the same.
That is, even when the clutch is under its inoperative condition (viz., the clutch is not applied with a hydraulic pressure), high speed rotation of the clutch drum caused by rotation of the high speed part tends to induce an engaged condition of the associated clutch unit due to the inevitable centrifugal force applied to the oil in the clutch drum. As is known, this phenomenon causes malfunction of the transmission, and in a severe case, induces seizure of the clutch plates of the unit.